Automatic volume control with noise suppression



22,1938. wff/AN E. ROBERTS 2,11%735 AUTOMATIC VOLUME CONTROL WITH NOISESUPPRESSION original Filved-Dec. 19, 1928A f6 Y a: alalr-J v20 cordancewith the amplication. For instance, trol coil 4 is less than acertainamount and a Nin-:o STA-Tes Param* cries AUTOMATIC' VOLUME CONTROL WITHNOISE f SUPPRESSION Walter van B. Roberts, vPrinceton, N. J., assignorto Radio Corporation of America, a

corporation of Delaware Application December 19, 1928, Serial No.326,990 Renewed September 18,l 1931 29 Claims. Y (Cl. Z50- 20) It iswell known that in radio receivers having In Fig. 4 is shownschematically a complete automatic yvolume control an annoying hissocradio receiver embodying the present invention, curs when, in tuning,the dial passes over freas applied specically to Fig. 2. quency bandscontaining no signal energy. This In the following matter which is givensolely 5 is due'undoubtedly, to the fact that in such defor purposes ofclearly describing the invention 5'VV vices, amplification automaticallybecomes very and not for purposes of limitation, speciiic exgreat in theabsence of signals with the result amples will be given in order that aclearer unthat static and other noises are amplified to a derstanding ofthe invention will be possible; great degree. f however, in no case isit to be taken that the par- This, of course, is inherent due to thevery naticular. values chosen or given by way of illustra- 10 vture andway that the said volume controls option areintended as a curtailment onthe scope erate, that is, the less energy coming into the re- Y of theinvention, except as set forth in the claims. ceiver, the greater thetendency to amplify. In Figure 1, device 3 represents diagrammati- Insome automatic volume control arrangecally a magnetron tube of wellknown type havments, such, for example, as disclosed in patent ing ananode surrounding an electron emitting 151 of T. A. Smith, entitled AnAutomatic gain confilament within an enclosure and Il represents itstrol, Patent No.l 1,999,668, issued April 30, 1935, external controlcoil. This tube is so constructthe plate current supplied 'to the tubesin the ed that it presents a relatively low direct current radiofrequency amplifying` system varies in acinternal resistance when thecurrent in its conin certain particular devices of this type the curmuchgreater internal resistance when the current supply to the radiofrequency tube is about rent in the said control coil exceeds a certain15,mils when very Weak signals are received, amount; that is, the deviceis constructed so as somewhat more than 15 mils when no signals are tobe very sensitive to changes in current passing present, and less than15 mils when loud signals through the control coil. The coil Il, throughare present; that is, approximately' `inversely leads I-I, is connectedin the plate current cirproportional to the strength of the signal.This, cuit of the radio frequency ampliiier of the reof course, is trueonly within certain limits. ceiving apparatus, not shown. Tube 2represents The broad system of noise suppression in autothe audio outputtube of the radio receiving apmatic gain control systems is disclosedand paratus and as is evident from the drawing the 30 claimed in anapplication filed by Arthur F. Van plate supply 5 of the audio tube isin series with Dyck et al., Serial No. 465,852, filed July '7, 1930. themagnetron tube 3 so that the magnetron tube The purpose of the presentinvention isv to conacts as a current limiting relay or control valvestruct a device for eliminating the above undefor the plate supply oftube 2. A loudspeaker or siredV effects without in any way impairing theindicator B and a by-pass condenser l are con- 35 operativeness andeffectiveness of the volume nected in the output 'circuit in the mannershown. control device. It is thus seen that as the magnetron tube 3 isAnother purpose of the invention is Vto utilize in series with the platesupply 5 of the audio a vacuum tube in its various capacities foracamplifier output tube 2 it very substantially de- 40 complishing thedesired results. creases the plate current of the audio output 4o Stillother purposes will appear in the descrip- When the current in winding 4exceeds a certain .tive matter which is to follow when read inconamount, say in this case 15 `mils, and that it nection with thedrawing in whichf allows normal current to flow to the plate of tubeFigure 1 illustrates diagrammatically one em- 2 When the current throughi is less than 15 mils bodiment of my invention in which a vacuum andthat by suitable adjustments the current lim- 45 tube of the type knownas a magnetron is used iting control may be made quite critical. The forthe control resistance, current in winding 4 being in the plate currentFigure 2 illustrates diagrammatically a mode supply of the radiofrequency tubes has its value of accomplishing the objects of myinvention by determined by the automatic volume control. the use of atriode tube and suitable biasing The operation of the device in Figure 1is as 5o means; follows: When no signal is present the volume Figure 3diagrammatically illustrates another control tends to increase theamplification which embodiment of my invention in which an osclresultsin great amplification of static and other lation circuit is made use offor the control undesired noises thus causing more current to go means.through coil 4 which in turn controls tube 3 in 55 Ill) such a way thatthe audio frequency end of the circuit is made substantially inoperativewhereby the undesired noises will not be heard through the phones 6.

If desired, a supplementary biasing direct current through 4 may besupplied to make the magnetron cut off at the desired eld strength, or aseparate control coil may be preferably provided for the biasingcurrent.

In Figure 2, I 2 is a triode having biasing means such as a grid batteryI0. The plate current of the volume controlled radio frequency tubes(not shown) goes from B+ through resistance I4 to the plates as shown bythe arrow. Resistance I4 is arranged so as to be in the input circuit ofthe triode I2 so that changes in potential drop across the ends of theresistance I4 due to an increase in the plate current of the radiofrequency tubes will affect the bias upon the grid of tube I2. BatteryI0 is of such voltage that it just shuts oi plate current throughresistance I3 when the radio frequency plate current owing throughresistance I4 has the assumed critical value of 15 mils. Tube I5 is thelast audio stage of the receiver, and terminals Iii-I6 are the inputterminals to the audio transformer I'I. A suitable grid bias source,such as battery I9, a plate current supply I8 and a telephone receiverdevice I'I are connected to the tube I5 as shown.

In this modication it is seen therefore that a normal flow of current(15 mils or less) through I4 will cause no change whatever in thereceiver. However, suppose that due to the absence of signals theautomatic volume control functions in such a way that a current say inexcess of 15 mils flows through I4. The potential drop thus producedacross resistance I4 opposes the grid biasing battery lil potential andcauses the grid to become sufficiently less negative to permit somecurrent to ow through resistance I3. This in turn causes more bias onthe grid of audio tube I5 which, may be any one or all audio tubes ofthe circuit, (only one being shown here for convenience). Due to theamplifying action of the tube I2, an increase in the radio frequencyplate current to 16 mils, for example, can readily be caused to increasethe negative bias of tube I5 sufciently to cut off all the platecurrent. A decrease, say to 14 mils however, would make no difference asthere is no current at all through I3 except when the radio frequencyplate current is greater than l5 mils.

In the modification shown by Figure 3, 25 is a triode having its inputand output coupled in any well known manner, for example by the coupling22, to produce oscillations under certain conditions. 'Ihe plate currentfor the radio frequency tubes (not shown) goes through from B+,resistance 24 to the plate electrodes of the radio frequency tubes insubstantially the same manner as shown in Figure 2. Coil 2|, coupled tocontrol grid coil 22 through the anode coil 22', rectifier 2, resistance28 and filter 29 constitute a rectifier circuit having its input coupledto the oscillatory circuit 23 of tube 25. The rectier is showndiagrammatically as a crystal 21 although any type of rectifier may beemployed. The current smoothing circuit or lter unit 29 is included sothat a smooth direct current is derived from the rectier. Resistance 28is supplied in the rectier circuit in series with the grid circuit 30-30of one or more of the audio frequency tubes of the circuit. A source ofplate voltage 26 is included in the plate circuit of tube 25.

l' It is evident from the figure that when the radio frequency amplifierplate current becomes excessive, say above 15 mils, more current willflow through resistance 24 thereby increasing the potential drop, orvoltage, across said resistance making the grid of tube 25 become lessnegative due to the fact that the potential across the resistance 24overcomes the eifect of battery 23 when the current through 24 reaches acertain value. When the effect of battery 23 is overcome, strongoscillations will be initiated in tube 25. These oscillations are pickedup by coil 2l, rectied by rectifier 2'I and smoothed out by the lter 29.The resulting direct current is passed through the resistance 28 thusgiving a biasing voltage which is applied in series with the grids ofone or more of the audio frequency tubes which in turn will shut off theoutput, as in the case of Figure 2.

WhenV this current is properly proportioned with respect to thedifferent biases the audio frequency plate supply will be entirely, orsubstantially cut off, with the result that the undesired noises areprecluded from being sent through the telephones, or other soundreproducing devices.

In the three modifications shown, the audio output should be shut off,or greatly diminished, onlyrwhen the plate current of the radiofrequency tubes increases above a certain predetermined amount (say 15mils) in average value averaged over a reasonable time.

To insure this, in any of the modifications, a low pass lter may beinserted in the connection between the radio frequency plate circuit andthe means for shutting off the audio frequency tube output, as shown byway of illustration at Figure 2, by network II which device will allowany temporary rise in current resulting from an unusually greatamplitude modulation, as for instance, in the case of a loud note inbroadcasting a musical selection, to be by-passed.

Fig. 4 is a schematic diagram of a complete receiver embodying theinvention, and it is to bc clearly understood that the diagram is purelyconventional, or functional, in nature. The input of the radio frequencyamplier includes a tuner, and the latter is symbolized by the usualvariable condenser representation. The detector is fed with theamplified radio frequency energy; it detects the energy, and transmitsthe detected energy to an audio frequency amplifier.

The block designated Automatic volume control symbolizes the arrangementfor automatically regulating the gain of the radio amplifier. As statedabove, its specic form may follow the teachings of the aforesaid Smithpatent. The audio frequency amplifier control, or background noisesuppressor, is functionally shown as being fed by a portion of the radioamplifier output; the output of the suppressor being employed to controlthe amplification efficiency of the audio amplifier. The schematicrepresentation of the background noise suppressor generally illustratesthe functions of each of the modifications in Figs. 1, 2, 3. Theresistor I4 is the resistor shown in the grid circuit of tube I2 of Fig.2. As shown in the aforesaid Smith patent, the automatic volume controlfunctions to vary the flow of radio frequency amplier plate currentthrough resistor I4. The latter is to be replaced by coil 4, or resistor24, when the system is used in connection with the arrangements of Figs.1 and 3 Arespectively. f

reproduction of undesired background noise impulses, which impulses tendto be greatly amplified by the radio amplifier whenever the collectedsignal energydecreases in intensity below the aforesaid minimum level.

Having thus described my invention what I desire toprotect by LettersPatent is as follows:

1. In a radio receiving circuit having an automatic volume controldevice, an intermediate space discharge device circuit having its input,

coupled to the output of one of the radio fre-- quency amplifier stagesof said receiving circuit, and its output coupled to the input of anaudio frequency vamplifier stage thereof, means in said intermediate,space discharge device circuit for normally maintaining'said device inan inoperative state, means for overcoming said first named means uponan abnormal increase in output current' in said radio frequencyamplifier whereby said devicel is "rendered operative and meanscontrolled thereby for limiting the amplifying characteristics of saidaudio frequency amplifier stage.

2. In radio receivingapparatus having automatic volume control means, anelectron disi charge device having an input circuit and an outputcircuit, said input circuitforming a portion of the radio frequencyamplifier output of said receiving apparatus and said output circuitforming a part of the audio frequency input circuitof said receivingapparatus, biasing potential means for said device and means insaidinput circuit for overcoming said biasing vpotential when the current insaid radio frequency amplifier output'v eX- ceeds a certainpredetermined value only when i sustainedv over an appreciable'length oftime.

3; In a radio receiving circuit having aradio frequency amplifier anaudio frequency amplifier,

and an automatic volume control device, a tube circuit having its inputresistively coupled to the plate supply of said radio frequencyamplifier through a low pass filter and its output resistively coupledto the input circuit of said audio frequency amplier and a biasingdevice in said tube input circuit for normally maintaining said tubeinoperative. l

4. In a multi-stage receiving circuit having an automatic volume controldevice, auxiliarymeans for coupling any stage thereof to any succeedingother of said stages comprising a normally inop-V erativespacevdischarge device-circuit having an input circuit comprising abiasingmeans anda resistance in series, and an `output circuitcomprising a resistance and a plate current supply, said resistancesbeing utilized for coupling said Space discharge device circuit, betweensaid other stages respectively and means whereby when a currentexceeding a predetermined value ows through said rst mentionedresistance for a sustained predetermined time interval said spacedischarge device will be rendered y operative whereby a current willflow through said second resistance and means controlled thereby forcontrolling the output of the stage connected thereto.

5. A signalling system comprising a high frequency amplifier circuit,means for tuning said circuit through a range of frequencies, a gaincontrolV device for regulating the sensitivity of said amplifier,electron discharge tube means for utilizing the amplifier output `andadditional means comprising an electrondischarge device forautomatically decreasing the utilizing efficiency of said last meanswhen said amplifier is adjusted to maximum sensitivity but is tunedthrough a point of said range where less than a predetermined amount ofhigh frequency energy is received.

6. A signalling system comprising a high frequency amplifier circuit,means for tuning said circuit through a range of frequencies, a gaincontrol device for regulating the sensitivity of said amplifier, meansfor changingv the frequencies amplified at high frequency to a lowfrequency, a low frequency amplier-andadditional means including a spacedischarge relay for automatically impairing the transmission einciencyof the low frequency amplifier when the high frequency amplifier isadjusted to'maximum sensitivity but is tuned through a point of therange where no high frequency energy is received.

7. A receiving system comprising a signaltranslating tube device, aradio frequency amplifier coupled to said device, means for regulatingthe sensitivity of said amplifier inversely with received signal energylevel and means comprising an electron discharge relay for disablingsaid tube device when said amplifier-is adjusted to maximum sensitivityby said regulating means and no signal energy is received.

8. In combination, a radio receiver comprising,

a radio frequency amplifier and a low frequency amplifier, means forcontrolling the amplification factor of the radio vfrequency amplifierand an electron discharge relay arranged to be actuated by variations inthe output of said radio frequency amplifier for reducing thetransmission efficiency of the low frequency amplifier when the gain ofthe radio frequency amplifier is adjusted to a maximum and no carrierwave is received. 9. In combination, a radio receiver comprising, aradio frequency amplifier, a frequency changing means and a lowfrequency amplifier, means for controlling the gain of the radiofrequency amplifier and an electron discharge device arranged to beactuated by variations in the output of said radio frequency amplifierfor reducing the transmission efliciency of the low frequency amplifierwhen the gain of the radio frequency amplifier is adjusted to a maximumand no carrier wave is received, said electron discharge device beingconnected between the radio frequency amplifier and the said lowfrequency amplifier.

10. In a signalling system which includes an amplifying circuit and again control device for regulating the sensitivity of the amplifier withtube means for utilizing the amplifier output, in

combination, an electron discharge device, re-

sponsive to a predetermined energy variation in the amplifying circuitoutput, for automatically decreasing the utilizing eciency of the lasttube means when said amplifier is adjusted to maximum sensitivity andwhere less than a predetermined amount of energy to be amplified isavaildischarge relay, responsive to a predetermined energy variation inthe high frequency amplifier output, for automatically impairing thetransmission efficiency of the low frequency amplifier when the highfrequency amplier is adjusted to maximum sensitivity but is tunedthrough a point of the range where the intensity of the incoming highfrequency energy is below a predetermined value.

12. In combination with a radio receiver including a high frequencyamplifier, a detector, a lower frequency amplifier, and an arrangementfor automatically regulating the gain of said high frequency amplifier,a device for automatically suppressing the reproduction of undesiredbackground noise impulseswhen the sensitivity of said high frequencyamplifier is increased above a predetermined level by said regulatingarrangement, said device comprising a circuit connected to said lowfrequency amplifier, said circuit including means, responsive to saidincreased sensitivity, for controlling the bias of an input electrode ofsaid low frequency amplifier.

13. In a radio receiver, in combination, an audio frequency amplifier, adetector connected to the input of the amplifier, a high frequencynetwork connected to the detector input, an automatic volume controlcircuit for regulating the transmission efficiency of said network insuch a manner that the radio frequency input to the detector issubstantially constant regardless of signal carrier intensityfluctuations in the input of said network, and means responsive tovariations in the magnitude of t'ne received signal energy for adjustingthe bias of an input electrode of said audio amplifier in a sense suchthat the audioamplifier output is greatly diminished when the signalenergy magnitude decreases below a predetermined intensity level.

14. In a radio receiver, in combination, a radio frequency amplifier,means for automatically adjusting the gain of said amplifier, saidreceiver additionally including a rectifier and a low frequencyamplifier in a network following said radio amplifier, and an auxiliarymeans, operative upon adjustment of said radio amplifier to maximumsensitivity and when signal energy below a minimum intensity level isimpressed on said radio amplifier, for automatically biasing a controlelectrode of said network to cut-off.

15. A radio signalling system of the type including a resonant highfrequency transmission network, tuned to a desired frequency, ademodulator and an audio amplifier, means for automatically regulatingthe transmission efficiency of said network in such a manner that thesignal energy input to the demodulator is at a substantially constantlevel, and additional means, responsive to a decrease of the said signalinput below a minimum level, for automatically biasing the audioamplifier in a sense such that the transmission efiiciency of the latteris substantially impaired.

16. In combination with an audio frequency amplifier having a controlelectrode and a source of audio signal frequency energy therefor, aspace discharge device having at least one electrode electricallyconnected with said source in such a manner that a decrease in intensityof said energy below a minimum level results in a predeterminedvariation in direct current potential of said electrode, andanadditional electrode in said device connected to a transmissioncontrol electrode of said amplifier, and responsive to said potentialvariation, for impairing the transmission efficiency of said amplifierwhen said energy decreases below said level.

17. In combination with the audio frequency amplifier of a radioreceiver, a background noise suppressor tube having its input electrodesconnected to a point in said receiver preceding the input of saidamplifier, the output electrodes of the tube being connected to theinput circuit of said amplifier, means in the said input circuit forestablishing a normal potential difference between the amplifier inputelectrodes such that the amplifier isl eflicient when audio signalenergy is impressed thereon, additional means, connected to the outputelectrodes of said tube, for providing a potential difference betweensaid amplifier input electrodes adapted to bias the amplifier to cutoff, and means connected to the suppressor tube input electrodes fornormally biasing the suppressor tube to inoperative state as long assaid signal energy is impressed on said amplifier.

18. In a radio broadcast receiver provided with a plurality of electrondischarge tubes included in successive stages of the receiver, means fortuning the receiver through the broadcast tuning range, automatic volumecontrol means for maintaining the audio output level of the receiversubstantially uniform regardless of signal carrier fluctuations, anauxiliary control arrangement adapted to operate when said tuning meansis adjusted to a setting at which broadcast signal energy below aminimum intensity level is collected, said control arrangement includinga biasing means, responsive to increased receiver sensitivity below saidminimum level, for biasing at least one of said tubes to cut-off.

19. A method of radio frequency signal reception which includes thesteps of collecting signal energy, amplifying the amplitude of thecollected energy, regulating the amplification of said collected energywith a portion of the amplified energy in a direction to opposevariations in the intensity level of the carrier of the collected signalenergy, detecting the amplified energy, amplifying the detected energy,and regulating the amplification of the' detected energy with a portionof the said amplified collected energy in a direction opposite to thatof said first regulation when the said level decreases below a desiredminimum. value.

20. A method of radio frequency signal reception which includes thesteps of collecting signal energy, amplifying the amplitude of thecollected energy, regulating the amplification of said collected energywith a portion of the amplified energy in a direction to opposevariations in the intensity level of the carrier of the collectedenergy, rectifying the amplifier energy, amplifying the rectied energy,and regulating the amplification of the rectified energy with a portionof the said amplified collected energy in a direction opposite to thatof said first regulation when the said level decreases below a desiredminimum value.

21. In a multi-stage radio receiver having an automatic volume controldevice, an auxiliary coupling circuit comprising an electron dischargedevice having its input coupled to the output of one of the stages ofthe radio frequency amplifier and its output coupled to the input of oneof the stages of the audio frequency amplifier of said receiver andmeans upon the attainment of a predetermined condition in the output ofsaid radio frequency stage for rendering said audio frequency stageinoperative.

22. In a radio receiving circuit, an auxiliary space discharge devicecircuitl having its input coupled to the radio frequency amplifieroutput and its output coupled to the audio frequency amplier input ofsaid radio receiving circuit, said audio input being also coupled tosaid radio frequency output, means in said radi'o frequency output foraffecting the conductivity of said auX- iliary space discharge deviceand meansv controlled thereby for controlling the output of said audiofrequency amplifier.

23. In a radio receiver, automatic volume control means actingautomatically to decrease the sensitivity of the receiver with increasein signal energy, a uni-lateral current device in the signal path ofsaid receiver having at least two electrodes, and means controlled inoperation by the automatic volume control means for biasing theelectrodes of said uni-lateral device in a direction toV decrease thesensitivity of the receiver when the signal energy is below apredetermined intensity level.

24. In a radio receiver, a uni-lateral current device in the signal paththereof having at least two electrodes, means for biasing the electrodesrelative to each other in a direction, and to an extent, such as toprevent the production of undesired noises by the receiver, and meansresponsive automatically to received signal energy for varying said biasin a direction to increase the sensitivity of the receiver to thereceived signals.

25. In a radio receiver, automatic volume control means, a uni-lateraldevice in the signal path of theV receiver having at least twoelectrodes, means for biasing the electrodes relatively to each other,and means controlled by the rendering the transmission of detectedsignals through the network inefficient when the received carrieramplitude decreases below a predetermined value.

27. In the operation of a high gain wave transmission system,'the methodof preventing the transmission of currents inadvertently arising Withinthe system which comprises normally biasing said system to suppresstransmission therethrough, and automatically removing said suppressionbias when the received waves attain a predetermined critical value.

28. In an electrical wave transmission system, the combination with aplurality of radio frequency and audio frequency stages cascaded to forma transmission line, means for controlling the gain of said stages tomaintain the signal voltage across predetermined points in said line ata substantially xed value over a wide range of received signal voltages,and means operative when the received signal voltage drops to a criticalvalue to impress upon an audio frequency stage following saidpredetermined points a bias Vvoltage which blocks transmission throughsaid .following stage.

29. In a radio receiver, a carrier-current amplier, a non-rectifyingmodulation-current amplier responsive to signals from saidcarrier-current amplifier, means dependent vupon the intensity of saidcarrier current for automatically maintaining the output of saidreceiver substantially constant over a range of received carriercurrentintensities, and means dependent upon the intensity of saidcarrier-current for rendering said modulation-current amplifierineiective, thereby reducing the output of said receiver substantiallyto zero, when the intensity of said carrier-current falls below apredetermined level below said range.

WALTER VAN B. ROBERTS.

